Abstract
Through investigating and comparing the fatigue behavior of an as-forged Mg-6.7Zn-1.3Y-0.6Zr (wt.%) alloy before and after solid solution treatment (T4) in laboratory air, the effect of T4 treatment on fatigue crack initiation was disclosed. S-N curves illustrated that the fatigue strength of as-forged samples was 110 MPa, whereas the fatigue strength of T4 samples was only 80 MPa. Observations to fracture surfaces demonstrated that for as-forged samples, fatigue crack initiation sites were covered with a layer of oxide film. However, due to the coarse grain structure and the dissolution of MgZn2 precipitates, the activation and accumulation of {10–12} twins in T4 samples were much easier, resulting in the preferential fatigue crack initiation at cracked twin boundaries (TBs). Surface characterization demonstrated that TB cracking was mainly ascribed to the incompatible plastic deformation in the twinned area and nearby α-Mg matrix.
Highlights
Extensive research work indicated that I-phase (Mg3Zn6Y, icosahedral quasicrystal structure, quasi-periodically ordered) strengthened Mg-Zn-Y-(Zr) alloys exhibit superior mechanical properties at both ambient and elevated temperatures[1,2]
No MgZn2 precipitates are observed because their size is less than 1 μm and much smaller than the voxel size used in X-ray computed tomography (XCT) analysis[18]
It is well known that the fatigue lifetime of engineering materials during a high cyclic fatigue testing is mostly determined by crack initiation process[26,27]
Summary
Extensive research work indicated that I-phase (Mg3Zn6Y, icosahedral quasicrystal structure, quasi-periodically ordered) strengthened Mg-Zn-Y-(Zr) alloys exhibit superior mechanical properties at both ambient and elevated temperatures[1,2]. A proper example can be seen in an as-forged-T5 Mg-Zn-Y-Zr alloy, where the fatigue strength at 107 cycles was as low as 50 MPa given the initiation of fatigue cracks at subsurface or surface inclusions[7]. Wang et al reported that a solid solution treatment (T4) could simultaneously induce grain growth and dissolution of MgZn2 precipitates for an as-forged Mg-Zn-Y-Zr alloy[18]. Few relevant work about the effect of solution treatment on fatigue strength degradation can be referred[6] It remains unclear whether a transition of fatigue crack initiation mechanism due to the T4 treatment exists. Based on the above description, the present work aims to investigate the effect of microstructural changes derived from solution treatment on the fatigue behavior of an as-forged Mg-Zn-Y-Zr alloy. The underlying fatigue crack initiation mechanism and possible transition for the alloy before and after T4 treatment will be discussed in detail
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